This invention relates to means and methods for reading bar code data formed on a highly reflective surface and/or where the width of the bars of the bar code vary widely along the length of the bars and/or where there is very little apparent contrast between the "bars" of a bar code and the surrounding space.
Bar code readers are used to sense the light reflected from a bar code, where the bar code normally includes a series of parallel black bars of different widths and spacing formed on a white background (or, vice versa, white bars formed on a black background). Typically, when the bar code is illuminated, the white or lightly colored regions reflect a large portion, if not all, of the incident light while the black bars absorb a large portion, if not at all, of the incident light. The resulting differences (contrast) in the levels of light reflected from the bar code enables the bar code to be read visually or sensed electronically.
However, a problem exists where the bar code is formed on a highly reflective surface, such as the top side of a silicon wafer which is highly polished and functions like a mirror. Light incident on the wafer surface in a normal direction is fully reflected since the surface acts like a mirror and light incident on the "data bars" formed on the silicon surface is also reflected with little attenuation, as shown for the depressed area 12 in FIG. 1A. Consequently, there is little contrast between the light reflected from the bars and the light reflected from the surrounding area. Hence, it is very difficult to read or sense the bar code. Furthermore, the high reflectivity levels from the bars and the surrounding area cause the photosensors to saturate and the bar code information to be lost.
The problem of reading bar codes formed on a highly reflective surface, such as silicon wafers, may be better understood by noting that the bar codes present on the wafer surface may be produced by forming holes (i.e., 12 in FIG. 1A) in the wafer surface by means of a laser or other appropriate hole forming means. Each "data bar" of a bar code to be sensed, as shown in FIG. 1B, is actually formed of a series of circular holes formed along a line rather than a well defined rectangular shape. The circular holes are formed of, and within, the silicon material and light incident in the normal direction (i.e., perpendicular) to the silicon surface will be reflected from the hole surface (which is reflective) without significant attenuation. Hence, as noted above, when viewed from a "normal" direction there is minimal contrast between the "data bar" (i.e., a hole or damage caused to the surface of the silicon wafer) and the surrounding space.
Another problem is that the data bar may be formed with circular holes at various densities ranging from circles with considerable overlap to a condition where there is some space between circular holes. Consequently, the data bars do not have a uniform width in a direction perpendicular to the length of the data bar, as is normally the case with typical bar codes. This should be evident from an examination of FIG. 1B which shows a very small cross-section at the point (X1--X1) of tangency or abutment and a substantially greater cross-section along the midpoint (X2--X2) of each circular hole. To accurately decode the presence of bar code data the data bar must have some minimum width. Also, the information contained in bar codes is often a function of the ratio of the width of the "data" bars to the space between the data bars. Where the width of the data bar varies considerably along its length, reading the data bars perpendicular to their length will produce different results depending where along the length of the bar the reading is taken (or made); (i.e., readings are position dependent). Hence, where the width of a data bar varies greatly along its length, reading a "data bar" in a conventional manner (i.e., in a direction perpendicular to the bar) makes the accurate decoding of a bar code difficult.
The problems discussed above are significantly reduced in systems embodying the invention.